1. Field of the Invention
The present invention relates generally to a power converter, and more specifically, to a power converter controller having a switch controller operating with a variable reference voltage.
2. Description of the Related Art
Power converters typically require error circuitry that provides an “error” signal between the output voltage of the power converter and a reference voltage, in order to regulate the output voltage. The error circuitry should provide a magnitude and a sign (positive or negative) of the output voltage relative to a reference voltage, so that the power converter can use such error signal to properly regulate the output voltage by increasing or decreasing the amount of power delivered to the output of the power converter in response to such error signal.
Conventional power converters typically generate an error signal by sensing the output voltage as an analog value, deriving the difference between the sensed output voltage and the reference voltage as an analog value and amplifying it. Conventional power converters may also use an analog-to-digital converter (A/D converter) to generate the error signal depending upon the control scheme. Other conventional power converters may use analog error amplifiers to generate the error signal.
In conventional switching power converters, a reference voltage compared with the sensed output voltage is generally fixed to a chosen voltage so that the output voltage of the power converter is regulated to a target level. Some power converters generate another reference voltage that is sometimes referred to as a “knee voltage” that is generally lower than the reference voltage. Such reference voltage and knee voltage of a preceding switching cycle may be compared with the sensed output voltage to generate a pulse signal in the next switching cycle.
By fixing the reference voltage and the knee voltage, however, it may be difficult to obtain a stable and robust regulation of the output voltage of the power converter. During certain switching cycles of the power converter, the sensed output voltage of the power converter may fluctuate well above or below the fixed reference voltage and the fixed knee voltage. For example, during the startup of the power converter or during under-regulation, the output voltage of the power converter may stay below the fixed reference voltage or the fixed knee voltage. Conversely, when the power converter is over-regulated, the sensed output voltage of the power converter may rise well above the fixed reference voltage. In such cases, the fixed reference voltage and the fixed knee voltage may no longer be used to determine the proper on-time and off-time of the pulse signal of the power converter in the next switching cycle.
FIG. 4 is a diagram showing the waveforms of the sensed output voltage Vsense of the power converter sampled in the first switching cycle relative to a fixed reference voltage and a fixed knee voltage in a conventional power converter. In the conventional power converter, the reference voltage VREG and the knee voltage Vknee have fixed values. Therefore, unless the sampled output voltage Vsense falls within a range defined by the reference voltage VREG and the knee voltage Vknee as shown by Vsense 410 in FIG. 4, a proper pulse signal for the second switching cycle cannot be generated from the reference voltage VREG and the knee voltage Vknee. For example, at the startup of the power converter or when the power converter is under-regulated, the sensed output voltage Vsense 408 of FIG. 4 may rise above the knee voltage Vknee at TKR0 but stay below the fixed reference voltage VREG. In this example, the events of the sensed output voltage Vsense 408 rising above the reference voltage VREG and dropping below VREG are not detected. Under a more extreme case of under-regulation, Vsense may stay below the knee voltage Vknee so that even the events of Vsense rising above or dropping below the knee voltage Vknee may not be detected. Therefore, conventional power converters must use different algorithms depending on the levels of Vsense and available events to determine the pulse signal for the second switching cycle when VREG and Vknee cannot be used. Moreover, Vsense 408 may oscillate after the initial spike, dropping below the knee voltage Vknee for the first time at TKD0. Because the oscillation of Vsense 408 can be random, the event of Vsense dropping below the knee voltage Vknee may not be meaningful for controlling the power converter. Therefore, the conventional power converters using the fixed reference voltage VREG and the knee voltage Vknee is less stable and robust than desired.
Therefore, there is a need for a power converter that can consistently and robustly control its output voltage by varying the reference voltage. There is also a need for a power converter that can generate a knee voltage having a fixed voltage offset from the reference voltage regardless of the variations in the reference voltage.